Digital reduction: Newly described dinosaur had only one finger per stubby forelimb

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Large tyrannosaurs managed to attain their status in the dinosaur kingdom among theropods wielding just two fingers at the end of their rather reduced arms. But they are now outdone in the realm of digital reduction. In this group where three fingers is the norm, an unusual new dinosaur has come to light that had only one finger on each arm.

Discovered in the Upper Cretaceous Wulansuhai Formation in China, this 84-million- to 75 million-year-old alvarezsaur, named Linhenykus monodactylus, would have looked decidedly less terrifying than a Tyrannosaurus rex. It likely would have weighed in at only about 450 grams and had even punier front limbs.

The theropod group, to which modern birds and tyrannosaurs belong, has been full of evolutionary flourishes, including feathers, flight and some fearsome claws. And the new paper demonstrates the high variability in hand morphology alone. "The one-fingered Linhenykus shows us how extensive and complex theropod hand modifications really were," Michael Pittman, of the University of London’s Department of Earth Sciences and co-author of the new paper, said in a prepared statement. Birds, for example, have three digits, the phalanges of which are "partially fused" to help strengthen wings, the researchers noted in their paper.

The uni-clawed theropod was not actually a big surprise to the scientists who described it. Other alvarezsaurs have been found with two of their three fingers withered to nubs, on their way out by way of evolution. Researchers have previously proposed that these other minimally fingered theropods might have been using their one strong digit to dig for food. And after assessing the build of these creatures’ forearms, the scientists decided that the dinosaur features "resemble the biomechanical tool kit of extant mammalian diggers in hard substrates, such as the giant armadilloPriodontes."

Perhaps the most peculiar aspect of Linhenykus was not what it had on the end of its short arms, but when these single claws seem to have been around. As a basal alvarezsaur that came before others that had more digits—even if they were not always being used—the new dinosaur "demonstrates that manual evolution in alvarezsaur did not follow a simple linear trend," the researchers concluded.

This mysterious reemergence of cast-aside features can be seen in other areas of the animal kingdom. "Vestigial structures, like legs in whales and snakes, may appear and disappear seemingly randomly in the course of evolution," Jonah Choiniere, of the American Museum of Natural History and co-author of the study, remarked in a prepared statement.